470 



PLANT DISTRIBUTION 



Swamps serve very well to illustrate on a small scale 

 the effect of water on vegetation types. If one finds a 

 pond with deep water in the middle and gradually shelv- 

 ing toward the shore with a bottom and banks composed 

 of deep rich soil, he will find that the vegetation is dis- 

 tributed in more or less regular concentric zones (Fig. 

 136). The open water in the middle, if deep enough, 

 can support only floating plants. Outside this there will 

 probably be a zone of water lilies with their roots an- 



FiG. 137. — A Typical Submersed Aquatic Flowering Plant, Cera- 

 tophyllum. The very narrow divisions of the leaves arc especially 

 characteristic of the aquatic habit. Both leaves and stems contain 

 large air spaces. 



chored in the bottom of the pond but with their leaves 

 extending up to the top and there floating more or less 

 freely (Fig. 137). In this zone may also be found other 

 plants with long stems and submerged leaves. All these 

 plants have weak bodies and depend largely on the water 

 to support their upper free parts. Nearer shore is likely 

 to be a zone of tall rush-like plants such as bulrushes, 

 tule, cat-tails, etc. They extend to just about the line 

 where soil appears above the water. Then comes a zone 

 of swamp plants of various sorts such as sedges. This 



